TAMPA — Transit officials say they've done their best to ensure that projected costs for Hillsborough County's proposed light rail system won't rise dramatically in the future.
But cost estimates already have spiked since county commissioners agreed this summer to ask voters if they support a 1-cent sales tax to pay for rail, more bus routes and road work.
Projections by the county's Metropolitan Planning Organization put the construction costs for a new light rail system — the figure commissioners considered — at $70 million per mile.
Now, Hillsborough Area Regional Transit officials say the cost likely will be $85 million to $120 million per mile, based on an analysis issued two weeks ago that doesn't consider inflation.
"I think that's a consistent pattern and it's occurred in city after city," said Martin Wachs, who supervises transportation research at Rand Corp., a nonpartisan, nonprofit think tank.
Officials with HART, which will run the trains, say the earlier estimate was made largely using the experience of other cities. The new numbers reflect a more localized analysis that revealed needed bridges and land purchases that could prove costly.
It builds in big contingencies for cost overruns, they say. Still, they acknowledge that the best estimates will come later, when paths are selected and engineering work is done.
So how should voters view the numbers as they cast ballots?
"Our numbers are more conservative," said David Armijo, HART's executive director. "All this . . . analysis does is give you a back-of-the-envelope number."
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Rail planners often use the word "conservative" to suggest they are considering every precaution in calculating costs.
Critics argue their estimates prove to be anything but that, with actual costs ending up much higher than figures that are initially sold to the public. They quote multiple studies that argue costs for rail are consistently underestimated, and ridership is overestimated.
Rail advocates say that those studies are old and that transportation planners are now better at hitting their targets.
"Those have been debunked," said Bill Millar, president of the American Public Transportation Association.
He pointed to several bits of more favorable research. A common theme: that a fairer starting point for judging estimates comes after the engineering is done.
But that doesn't help voters, since those figures come long after they decide.
"It think that means people need to be cautious and skeptical of those figures," said Wachs, who has studied transportation projects and urban planning for three decades as a former professor at the University of California at Berkeley and UCLA.
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When MPO planners developed their projections in 2008, they looked at the actual cost of nine recent rail projects in six cities and adjusted for inflation.
HART initially estimated that the sales tax hike would spur $15 billion in transit spending over 30 years, including federal and state contributions. Of that, about half would go to build, run and pay debt on the rail system.
The first two lines would connect downtown Tampa to points north and west. Plans call for later extensions to South Tampa, Brandon and Westchase, with links to neighboring counties.
The new HART analysis projects capital costs alone for the first two legs of $2.8 billion.
"From what I've seen of their numbers, they seem to be very, very conservative," said Beth Alden, a multimodal systems group leader for the MPO who helped oversee the formation of the earlier projections.
The new estimates assume the first two legs will run farther than anticipated initially. The numbers reflect many unknowns, and HART officials say they have assumed worst-case scenarios as they seek to fill in the blanks.
Mary Shavalier, HART's planning chief, said there is reason to believe that cost projections could actually come down. Recent rail projects elsewhere were built when steel and concrete prices were rising rapidly, and construction is now less expensive in a weak economy.
The estimates also contain a contingency for 50 percent overruns on land costs. If the rail lines run on state land along Interstate 275, land costs would be nil. And if the route between downtown Tampa and West Shore Boulevard coincides with a planned interstate expansion, engineering and construction costs could be shared, she said.
Armijo, the HART executive director, noted that the last time Hillsborough got close to pursuing light rail in 2002, the cost per mile projection was roughly $40 million. His parenthetical question: What's the cost of pushing off rail until some date in the future?
"Delaying things will only raise the costs," he said.
Times researcher John Martin contributed to this report. Bill Varian can be reached at (813) 226-3387 or firstname.lastname@example.org.